An air battery is a battery employing oxygen as a cathode active material; at the time of discharge, air is introduced from outside the battery. So, compared with other type of batteries which incorporate active materials for both cathode and anode, it is possible to enlarge the occupancy rate of the anode active material in the battery case. Hence, in principle, such an air battery has features that dischargeable electric power is large and that downsizing and weight saving can be easily realized. In addition, oxidation power of oxygen to be employed as the cathode active material is strong so that the electromotive force is relatively high. Moreover, since oxygen is a clean resource the amount of which is not limited, the air battery is environmentally-friendly. As above, air battery has many advantages; therefore it is expected to be used for batteries for, for example, hybrid cars and mobile devices.
When water enters into an air battery using metal as the anode in emergency situations, the water and the metal tend to react. If the water and the metal react in this way, it is predicted that the air battery may be deteriorated. So, to inhibit deterioration of the air battery, presumably, it is important to detect the entering of water into the air battery quickly.
As a technique regarding such an air battery, for example, Patent document 1 discloses an air battery in which low-voltage alarm sounds when the detected voltage becomes equal to or less than the threshold level.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-209787